Global System for Mobile communications (GSM) is a Time Division Multiple Access (TDMA) system, whereby users are separated by different timeslots. This requires radio transmitters to continually switch between off and on states. The transients associated with switching cause radio energy to be emitted on frequencies beyond the channel that is allocated to the user, causing interference to other users and degrading the overall performance of the network. In order to minimise these emissions, the shape of the transient is typically controlled in the time domain via a gain control input to the power amplifier (PA). This process is known as ramping. The shapes of the ramps need to be adjusted so that strict specifications on power-versus-time (PvT) and switching spectra are satisfied at all power levels.
Similar considerations may apply to any communication system in which different signals are transmitted on the same physical channel but at different time slots, e.g., Time Division Duplex (TDD) systems.
In the field of wireless communications, and particularly wireless cellular communications, there has been increasing interest in recent years in the idea of moving more of the functions of the transceiver into software. A transceiver with a tendency towards software implementation is sometimes referred to as a software modem, or “soft modem”. The principle behind software modem is to perform a significant portion of the signal processing and other functionality required for the wireless communications in a generic, programmable, reconfigurable processor, rather than in dedicated hardware.